Low-coherence inferometric device for light-optical scanning of an object

ABSTRACT

Low-coherence interferometric apparatus for light-optical scanning of an object ( 18 ) with a low-coherence interferometer ( 6 ) comprising a low-coherent light source ( 7 ), a reference reflector ( 21 ) and a detector ( 25 ), wherein light emitted by the light source ( 7 ) is split into two optical paths ( 11,12 ), a first fraction of the light being irradiated as measurement light ( 16 ) onto the object and a second fraction of the light being irradiated as reference light ( 22 ) upon the reference reflector ( 21 ), and wherein, after reflection on the object ( 18 ) or the reference reflector ( 21 ) respectively, the measurement light ( 16 ) and the reference light ( 22 ) are combined at a beam junction ( 10 ) in such a manner that an interference signal which contains information about the reflection intensity of the measurement light, relative to the respective scan position is generated. 
     In order to enable a very fast scan, a variable wavelength selection device ( 30 ) is positioned in the light path of the detection light between the beam junction ( 10 ) and the detector ( 25 ). A wavelength-dependent selection of the detection light ( 24 ) is performed by this device in such a manner that the detector ( 25 ) selectively receives preferentially light with wavelengths which correspond to a predetermined sequence of wavenumbers k. For varying the scan position along the scan path ( 27 ) different sequences of wavenumbers k can be set.

The present invention refers to a low-coherence interferometricapparatus for light-optical scanning of an object by detecting theposition of light-remitting sites which are located, at variabledistances from the apparatus, along a scan path which extends in a scandirection (i.e. in the direction of the detection light beam;“z-direction”). Hereafter this is referred to as Low Coherence DistanceScan (LCDS).

Such apparatuses, and the corresponding methods are utilized forexamining a variety of objects. They allow to determine, with a highestlevel of precision, the distance to one or a plurality of reflectingsites of an object or to provide a tomographical picture. Significantareas of use are the automatic measurement of object surfaces andanalysis of the optical scattering behavior inside an object. The latteris especially significant in the medical field (tissue diagnostics).

In some applications it is sufficient to scan the objectonedimensionally, i.e. only along a scan path which extends in thedirection of the beam. Most cases of use, however, require to obtain, bymeans of an additional lateral scan, information about reflectingstructures in a scan plane or (three-dimensionally) in a volume segment.This requires a two- or three-dimensional scan, which, in the simplestcase, may be achieved by one- or two-dimensional lateral shifting of theinterferometer. Such methods allow generation of a multidimensionaltomographical picture and are commonly called OCT (Optical CoherenceTomography).

It is common to all LCDS methods that light of a low-coherent(spectrally broadband emitting) light source is split into two lightpaths, i.e. a measurement light path, which penetrates into the testspecimen, and a reference light path. Before striking a detector, bothpartial light paths are combined in such a manner that interferenceoccurs. To this end the apparatus contains an interferometer device,which, in addition to the low-coherent light source, usually contains abeam divider, a reference reflector and the detector. The light pathsbetween these elements form interferometer arms. The light from thelight source passes through a light source arm to the beam divider,where it is split. A first fraction of light is irradiated, asmeasurement light, onto the object in the scan direction, whilst asecond portion of light, as reference light, reaches the referencereflector via a reflector arm. Both light fractions are reflected (themeasurement light at light reflecting sites in the examined object, thereference light at the reference reflector) and travel back along thesame light path (object arm, reference arm) to the beam divider. Herethe light fractions are recombined and further transported as detectionlight via a detector arm to the detector.

During scanning, the longitudinal scan position is being varied in afast sequence. Normally, this is achieved by changing the relationshipof the path lengths of the reference light path and the measurementlight path. Thereby the position along the scan path is varied, forwhich the conditions for interference of the measurement light and thereference light are met (namely that the optical path lengths of bothlight paths differ by no more than the coherence length of the lightsource). At each point of time the present scan position is the positionon the scan path for which the optical length of the measurement lightpath is the same as the optical length of the reference light path (fromthe beam division to the beam junction; “Coherence condition”).Normally, the reference mirror is displaced in the direction of thereference beam, thereby reducing or increasing the length of thereference light path.

Further details about a plurality of known LCDS devices are described incorresponding literature sources, including the following publications:

-   1) WO 95/33971-   2) J. M Schmitt “Compact in-line interferometer for low-coherence    reflectometry”, Optic Letters 1995, 419 through 421.-   3) WO 97/27468.

The present invention refers especially to applications in which anextremely fast longitudinal scan is desired. An important example is thecontinuous examination of multilayer foils (multi-foils) for productionsurveillance or quality control. The foil passes with high speed along ameasurement head, and continuous control has to be applied to determinewhether a certain desired foil thickness (for example 100 μm) ismaintained within predetermined limits. Such cases of utilizationrequire a very high scan speed. Assuming, for example, a surface spotdiameter, to which the examination refers, of 8 μm and a travel speed of10 m/sec, measurement data would have to be recorded approximately every0.8 μsec. This corresponds to a minimum scan rate of 1.25 MHz. At 256spots per longitudinal scan, this results in a repetition rate of 4.9kHz. Such high repetition rates cannot be achieved by a mirrordisplacement.

Several proposals have been made to achieve a higher repetition ratewith LCDS devices.

In the publication

-   4) K. F Wong et al: “400-Hz mechanical scanning optical delay line”,    Optics Letters 1993, 558–560,    an optical retarding section is described, which may be integrated    into the reference arm of an interferometer. The variation of the    optical path length is achieved by a combination of an angular    dispersion grating and a mirror which is pivotable within a very    restricted angular range.

A similar device is also described in

-   5) U.S. Pat. No. 6,111,645 and-   6) G. J. Tearney et al: High-speed phase- and group-delay scanning    with a grating-based phase control delay line”, Optics Letters 1997,    1811–1813,    as part of a LCD device which is reported to be suitable for    extremely fast scanning. In these publications, the basic principle    used in citation 4) is generalized in the sense that a dispersion    grating should be utilized in connection with a spectral phase    shifter. Also non-mechanical possibilities for the realization of a    spectral phase shifter, especially an acousto-optical modulator    (AOM), are described.

A disadvantage of these proposals is the double passage of light throughthe retarding unit composed of the angular spectral grating and opticalphase shifter which requires a very difficult alignment, since a precisereentrance into a single-mode light conducting fiber is required.Additionally, a high loss of intensity is caused by this light path.

Additional efforts for solutions proposed by the prior art are discussedin the initial sections of citations 5) and 6):

-   -   A modification of the optical path length may be achieved by        piezoelectric fiber stretching. This, however, requires a        relatively large-sized unit and does not allow a sufficiently        high repetition rate. In addition, the energy consumption is        high.    -   The longitudinally moveable mirror in the reference channel may        be replaced by a pivoting glass cube (see also U.S. Pat. No.        6,144,456). This causes, however, a non-linear change of the        optical path length and a dispersion which depends on the        optical path length. Again, the achievable repetition rates do        not satisfy high requirements.

Based on this situation the technical problem addressed by the presentinvention is to provide an interferometric apparatus which allows, withtolerable expenditure, an extremely high repetition rate of longitudinalscanning.

This problem is solved by a low-coherence interferometric apparatus forlight-optical scanning of an object, by detecting the position oflight-remitting sites which are located along a scan path running in ascan direction, with a low-coherence interferometer comprising alow-coherent light source, a reference reflector and a detector, whereinlight emitted from the light source is split by a beam divider into twooptical paths and a first fraction of the light is irradiated asmeasurement light onto the object and reflected at a light-remittingsite located at a variable scan position on the scan path, and a secondfraction of the light is irradiated as reference light onto thereference reflector where it is reflected, the adjustable scan positionis varied along the scan path to perform a scan, and the measurementlight and the reference light are combined at a beam junction in such amanner that the resulting detection light, upon striking the detector,generates an interference signal which contains information about thereflection intensity of the measurement light relative to the respectivescan position, characterized in that a variable wavelength selectiondevice is positioned in the light path of the detection light betweenthe beam junction and the detector, by which a wavelength-dependentselection of the detection light is performed in such a manner that thedetector selectively receives preferably light with wavelengths whichcorrespond to a predetermined sequence of wavenumbers k, and differentsequences of wavenumbers k can be set for varying the scan positionalong the scan path.

Contrary to the above explained earlier efforts to achieve extremelyfast longitudinal scans, the scanning unit for setting the scan positionis integrated in the light path of the detection light downstream fromthe joining of the reference light and the measurement light. Accordingto the invention the variation of the longitudinal scan position is notbased on a change of the relationship of the length of measurement- andreference light paths, but on a selection of a sequence of wavelengthsof the interfering detection light. This selection is varied by means ofthe wavelength selecting device in such a manner that the sequence ofwavenumbers (“k-profile of the wavelength selecting device)corresponding to the selected wavelengths coincides with that k-profileof the interferometer which corresponds to the respective scan position.This will hereafter be explained in detail, based on the figures.

The physical phenomenon utilized in the invention has been known for along time as so-called “Müller stripes”. Occasionally, it was also usedin the context of interferometric methods. DE 4309056 describes thepossibility to determine the distance of light scattering sites, i.e.their intensity distribution in the direction of the detection beam, byspectrally decomposing the light by means of a spectral device, thespectrum being detected with a location-sensitive light detectiondevice, for example a row of photodiode cells. According to the documentthis arrangement allows to determine by Fourier transformation theintensity distribution of the detected spectrum. This method isinadequate for fast longitudinal scans, since by far too much time isrequired for data interpretation of the photodiodes and processing bymeans of a Fourier transformation. Additionally, the detector signal isquite weak in view of the required good local resolution. Therefore theS/N (signal/noise) ratio is bad.

Several important advantages are achieved by the invention:

-   -   A complete longitudinal scan may be accomplished with a very        high repetition rate (10–100 kHz). For many applications,        especially for the continuous inspection of moving objects, it        is important that an even higher scanning frequency per scanning        site (1–10 MHz) is possible.    -   The measurement head of the device can be miniaturized very        well, since the scanning unit is disposed in the detection light        path, which can be connected with the remaining portions of the        interferometer, which may be integrated into a compact        measurement head, by means of light conducting fibers.    -   Evaluation is not dependent upon phase-sensitive information in        the detection light path and is, therefore, quite robust. Also        the risk of signal distortions caused by misalignment is        relatively low.    -   The light intensity recorded by the detector is high (especially        as compared with DE 4309056), since no location-selective        detection is required.    -   In case that the optical dispersion in the measurement light        path is different from the dispersion in the reference light        path, this results with prior devices in a lack of signal        precision. In the context of the present invention, such        dispersion differences may be offset by correspondingly        adjusting the k-profile of the wavelength selecting device.

Hereafter the invention will be explained in more detail, based onexemplary embodiments shown in the figures. The features shown anddescribed may be used separately or in combination to create preferredembodiments of the invention. In the figures:

FIG. 1 shows a schematic representation of a LCDS apparatus according tothe invention,

FIG. 2 shows a schematic representation of a part of a first embodimentof a variable wavelength selection device,

FIG. 3 shows a schematic representation of a part of a second embodimentof a wavelength selection device,

FIG. 4 shows a diagram to explain the analog and digital selection bymeans of a spatial light selection device,

FIG. 5 shows a graphical representation of the superposition of twodifferent wavelengths,

FIG. 6 shows a graphical representation of the k-profile of aninterferometer when the measurement light is reflected by alight-remitting site located at a defined scan position,

FIG. 7 shows a schematic representation of a first embodiment of amechanically variable spatial light selection device,

FIG. 8 shows a schematic representation of a second embodiment of amechanically variable spatial light selection device,

FIG. 8 a shows an enlarged cutout of FIG. 8,

FIG. 9 shows a schematic representation of a part of a third embodimentof a wavelength selection device,

FIG. 10 shows a schematic representation of a part of a fourthembodiment of a wavelength selection device,

FIG. 11 shows a schematic representation or a part or a fifth embodimentof a wavelength selection device,

FIG. 12 shows a schematic representation of part of a sixth embodimentof a wavelength selection device.

The LCDS apparatus 1 shown in FIG. 1 consists of a measurement head 2, ascanning unit 3 and an electronic unit 4. The representation is not toscale and is strongly schematic. Constructive details which are notessential for the function of the invention are not shown.

The measurement head 2 and the scanning unit 3 contain the opticalcomponents of a low-coherence interferometer 6. The light of a lightsource 7 is coupled via a lens 8 into a single-mode light conductingfiber which forms the light source arm 9 of the interferometer 6. Theprimary light transported in light source arm 9 is equally divided bymeans of an optical coupler 10, acting as a beam divider, as measurementlight 16 into a sample arm 12 and, as reference light 22 into areference arm 11, in which arms the light transport also takes placeinside light conducting fibers. In the sample arm 12, the measurementlight 16 is coupled out by means of an objective 13, composed of lenses14 and 15. Lens 15 refocuses the measurement light 16 radiated throughaperture 17 towards a test specimen 18.

Both in sample arm 12 and in reference arm 11 a reflection takes place,namely at a light-remitting site 20 of the measurement object 18 and ata reference reflector 21, respectively. The reflected measurement light16 and the reflected reference light 22 are recombined in opticalcoupler 10 and are transported as detection light 24 in a detection arm23 towards detector 25.

Up to this point, the construction of the interferometer 6 isessentially conventional and therefore needs not be explained in moredetail. Instead of the shown interferometer device, another knownconfiguration can also be used. Especially, instead of the optical fiberversion using an optical fiber coupler 10, a free beam arrangement witha free beam splitter can be used. In principle, it is also possible touse separate optical elements as beam divider for light separation onthe one hand and as beam uniting elements on the other hand. Preferably,however, the same optical element 10 is used for beam division andjunction, as shown.

A specific feature of the interferometer device contained in themeasurement head 2 is that neither the reference arm 11 nor the samplearm 12 contain means, by which the lengths of both arms (generallyexpressed, the lengths of the measurement light path and reference lightpath) are changed relative to each other in order to vary thelongitudinal scan position along a scan section 27, shown in dottedlines in FIG. 1, in the scan direction symbolized by arrow 28. Ratherthe variation of the scan position, which is required to accomplish thelongitudinal scan, is produced by means of the scanning unit 3integrated into the light path of the detection light 24, between thejunction of the light (by means of the optical coupler 10) and thedetector 25.

Scanning unit 3 contains a variable wavelength selection device,generally designated 30, whose essential components may be seen moreclearly in FIGS. 2 and 3, in two different embodiments. In the preferredcase shown, it comprises a spectral separation device 31, by which thedetection light 24 is spatially separated, dependent on its wavelengthλ. In the case shown, the spectral separation device 31 is formed by areflecting spectral grating 32. However, also other optical elements(transmission gratings, prisms), commonly used in spectral devices maybe chosen. Spectrally separated light reflected from spectral grating 32is focused onto a spatial light selection device 38, by means of anoptical imaging system 36 which is composed of two objectives 34 and 35.The first objective 34 collimates light emitted from entrance pupil 37of the wavelength selection device 30 onto the spectral separationdevice 31, while the second objective 35 focuses light emitted from thespectral separation device 31 onto the light selection device 38.

The spatial light selection device 38 has light passage areas 39 andlight blocking areas 40, alternately disposed along a line, whichpreferably is straight and extends in a spatial direction which isdesignated x in the figures. In any case, the line of the alternatinglight passage and blocking areas 39,40 must extend transversally to theoptical axis A of detection light 24, such that light which, dependenton its wavelength, is spatially separated along the line by the spectralseparation device 31 strikes the alternating light passage and blockingareas in such a manner that it is transported towards the detector 25with alternating intensity, corresponding to the wavelength.

This may be achieved both with a transmission device, shown in FIG. 2and with a reflection device, shown in FIG. 3. Detection light 24 passesthrough the light passage areas 39 with less attenuation, as compared tothe blocking areas 40. For example, in FIGS. 2 and 3, light withwavelength λ₁ striking the central section of a light passage area 39reaches detector 25 practically without attenuation, whereas light withwavelength λ₂ centrally striking a blocking area is blocked nearlycompletely. Light striking with wavelengths λ₃ between a light passagearea and a blocking area, is partially attenuated. Based on FIG. 3, itbecomes apparent that the expressions “light passage area” and “lightblocking area” should not be understood, in a limiting manner, in thesense of a transmission device where light passes through an opticalelement. On the contrary, the desired alternating degree of attenuationmay also be caused by a reflecting optical element.

FIG. 4 shows that in both cases of light selection, i.e. thetransmitting spatial light selecting device 42 according to FIG. 2, aswell as the reflecting spatial light selection device 43 shown in FIG.3, the transmission T and reflection R, respectively, of the elementvaries, dependent on position x, preferably in analog (especiallysinusoidal) manner. Digital selection, shown in figure in dotted linesis, however, also possible. It is decisive that light selected accordingto the defined k-profile of the wavelength selection device 30 ispreferentially transported to the detector 25. Preferably, thedifference between the minimum light attenuation of wavelengthscorresponding to the k-profile and the maximum light attenuation of the“blocked” wavelengths (“selection contrast”) should be as large aspossible.

According to the embodiments of the invention shown in FIGS. 1 through3, the variable light wavelength selection, in accordance with thek-profile of the wavelength selection device 30, is achieved with aconstant angular dispersion of the wavelength-dependent lightseparation, in combination with a variation of the distance of thealternating light passage and blocking areas 39,40 of the spatial lightselection device 38. Alternatively, it is also possible (and explainedhereafter, based on FIG. 11) to use a spectral separation device 31 withvariable angular dispersion in combination with a constant spatial lightselection device 38. In principle, it is also possible that both theseelements are variable.

For example an electrically operated LCD mask may be used astransmitting variable spatial light selection device 38 (FIGS. 1 and 2).In this case, the minimum distance of adjacent transmission areas isgiven by twice the pixel distance of the mask. Larger distances may beadjusted stepwise as multiples of this distance. An approximately analogtransmission variation may be achieved if the pixel distance is muchsmaller than the shortest desired distance between the transmissionareas.

In this respect a reflection device of the type shown in FIG. 3, where aDMD (Digital Mirror Device) can be used as variable light selectiondevice, is especially advantageous. Such micromirror arrays are producedwith extremely small pixel distances, especially for projection systems.

Hereafter, the principle used according to the invention will beexplained, based on FIGS. 5 and 6.

FIG. 5 shows, along a path Δz, the superposition of two wave trains 45and 46, oscillating in phase at the origin (zero point). At the end ofpath Δz the wave trains are again in phase, thus constructivelyinterfering with each other. It can be directly derived from the figurethat two wave trains interfere constructively under the conditionsshown, when their wavelength is an integer fraction of Δz, i.e. when thecondition λ=Δz/n applies.

For the purpose of simplification, only two wave trains were consideredhere. In reality, an interference of many adjacent wave trains takesplace. By considering the correlation between wavenumber k andwavelength λ (k=2π/λ) the general rule may be derived that along a pathΔz those wave trains interfere constructively, whose wavenumbers differbyΔk=2π/Δz.  (1)

Such interference also takes places in the detection arm of aninterferometer. The origin zero point from which path length Δz ismeasured, is defined by the point of the measurement light path forwhich the optical path lengths of the measurement light path and thereference light path are identical. Hereafter, it will be designated“point of coincidence of optical lengths”. In the context of theinvention, the coincidence point is significant in two ways:

-   a) On the one hand, it marks the point for which the coherence    condition explained further above is met. In the common LCDS    devices, this represents the basis for longitudinal scanning.-   b) At the same time, it marks the position at which the measurement    and reference light are in phase for all wavelengths (provided there    are no differences of optical dispersion). The coincidence point is,    therefore, the zero point of the longitudinal scan according to the    invention. The actual scan position is located at a distance Δz from    the coincidence point.

FIG. 6 shows an interference spectrum resulting from such asuperposition (wavelength-depended intensity normalized to the maximumvalue) in case of a light source with a central wavelength λ_(o)=800 nmand a spectral band width Δλ_(FWHM)=50 nm for an interference pathlength Δz=100 μm.

Such a spectrum may be experimentally observed by placing a reflector inthe measurement light beam of a LCDS device according to FIG. 1 at adistance Δz from the coincidence point of the interferometer and byanalyzing, at the position of the spatial light selection device 38, theintensity variation in x-direction along line 55, i.e. the dependence ofintensity upon the wavelength (by means of a locally sensitive ordisplaceable detector). The k-profile of the interferometer in thek-space corresponds, for the chosen value of Δz, to this spectrum in theλ-space.

From the above equation (1) a direct relation of the distance of themaxima of the k-profile and Δz can be derived mathematically.Consequently, in k-space the points of maximum interference of thek-profile of the interferometer are equidistant, as long as it is notrequired to consider differences of optical dispersion between themeasurement light path and the reference light path. Setting of alongitudinal scan position at a distance Δz from the coincidence pointis therefore possible by setting the variable wavelength selectiondevice 30 to an equidistant sequence of wavenumbers k, whose distancesΔk are calculated in accordance with equation 1. Since the correlationbetween λ and k is not linear (rather reciprocal) the correspondingspectrum in λ-space is not strictly equidistant. When considering arelatively narrow band spectrum, as shown in FIG. 6 the sequence of theselected λ-values is, however, approximately constant, too.

As repeatedly mentioned, the preceding considerations are based on theassumption that no optical dispersion has to be taken into account, i.e.the dependence of the refractive index of the wavelength in themeasurement light path is the same as in the reference light path. Incommonly used LCDS devices, the spatial resolution of the scan signal isnegatively influenced by dispersion differences. Therefore considerableefforts are usually made to achieve, by an adequate choice of the lightconducting means, as much similarity of the optical dispersion of bothlight paths as possible. In the context of the present invention it is,however, possible to offset in a simple manner the dispersiondifferences between the light path of the measurement light 16 and thelight path of the reference light 22, by choosing the sequence of thewavenumbers k which are selected by the longitudinal wave selectiondevice 30, deviating from an equidistant sequence, in such a manner thatthe difference in dispersion is offset. In other words, the k-profile ofthe wavelength selection device is adapted to the k-profile of theinterferometer, which is not equidistant, considering the dispersion.Experimentally, this may take place in a relatively simple manner bypositioning a reflector in a plurality of different scan positions onthe scan path 27 and, for example as above described, measuring theresulting spectrum in the detection light path of the interferometer.According to this procedure a k-profile of the interferometer isobtained for each scan position within the Δz range. The same k-profilesare also selected by the wavelength selection device 30 and varied toaccomplish a longitudinal scan.

After passage of the wavelength selection device 30, selected light 24strikes the photosensitive surface of a detector 25. The detector 25 isnot locally selective, i.e. it transforms the entire light intensitywhich it receives into an electrical signal which is transferred to theelectronic unit 4, where it is evaluated. According to the preferredembodiment shown in FIGS. 1 to 3, a condenser lens 48 is arranged infront of the detector 25, acting as light collecting element 49. It isthus possible to capture, with a comparative small detector surface, theentire light which passes through the wavelength selection device 30.

Inside electronic unit 4, the intensity of light captured by detector25, is recorded by an evaluation unit 50, dependent on the setting ofthe k-profile of the wavelength selection unit 30. To each k-profile,the corresponding value of the scan position Δz is allocated. Theintensity of the measurement signal, after deducting a base line (i.e.the difference of the intensity from the base line signal), correspondsto the intensity of the reflection at the respectively selected scanposition.

Although scanning is not based on a modification of the relation of theoptical wavelengths of the measurement light path (measured up to thecoincidence point) and of the reference light path, this does not meanthat the position of the reference reflector 21 in the reference lightpath must be constructively fixed. On the contrary, for the purpose ofthe alignment of the device, it can be advantageous to make thisposition adjustable. During the scanning procedure, however, the lengthof the reference light path remains constant.

FIGS. 7 and 8 show two different embodiments of a light selection device38 which may be mechanically varied. A common feature of both is that ona pivotable disc 54 and 56, respectively, light passage and blockingareas 39,40 are provided in the form of stripes which extend in such amanner that their stripe distance, measured along a line 55 runningacross the disk surface, varies during rotation of the disk. The lightpassage and blocking areas may be produced in any desired shape, forexample by photo-lithographic processing of a metal-coated glass plate.

In the case of the disk 54 shown in FIG. 7 the light passage areas 39,40are straight and parallel. A line 55 which is effective regarding thewavelength selection (i.e. the line, upon which the spectrum of thespectral separation device is projected) runs in such a manner that theeffective distance of areas 39,40 varies when the disk 54 rotates.

In the embodiment shown in FIGS. 8 and 8 a, the light passage andblocking areas 39,40 converge at a border stripe 56 over sections oflength l towards each other in such a manner that their distance,relative to line 55, upon which the spectrum is projected, decreases ineach segment 57 during rotation. During the passage of a segment 57, acomplete scan takes place, so that a very high scan speed is achieved.For example, with a rate of 100 rotations per second and 100 segments 57(with replicated structures), a repetition rate of 10 kHz may beachieved. Since the curvature of the line structure of areas 39,49 maybe chosen freely, it is possible to adapt the light selection withrespect to dispersion differences of the measurement- and referencelights paths.

In connection with FIGS. 2 and 3, the possibility was already describedto utilize for the spatial light selection device 38 an optical element(LCD, DMD) which allows selective setting of transmission or reflectionin different partial sections of its surface by electronic means.Another example of this general principle is shown in FIG. 9. In thiscase the detection light 24 originating from the spectral separationdevice 31 is focused on the surface of an AOM (Acousto-OpticalModulator). Inside the AOM, continuous sound waves are generated. Theresulting vibrations in the crystal (which consists, for example, ofTeO₂) result in a spatial light selection under an angle whichcorresponds to the first order diffraction. Detector 25 and condenserlens 48 are arranged at this diffraction angle relative to the opticalaxis of light striking AOM 59. Contrary to the earlier describedembodiments, the light passage and blocking areas 39,40 of the spatiallight selection device 38 formed by AOM 59 are not stationary on itssurface, but move continuously in x-direction. The function of theinvention is, however, not negatively affected by this fact.

FIG. 10 shows that the optical imaging function, required in scanningunit 30, not necessarily has to be provided by additional constructionelements. For example an arcuate spectral grating 60 may be utilized asa spectral separation device 31. It provides not only the spectralseparation, but also the required collimation of the light coming fromthe entrance pupil 37 onto the spatial light selection device 38.

As already stated, FIG. 11 shows an alternative embodiment of thevariable wavelength selection device 30. Here a spectral separationdevice 31 with variable spreading is used in combination with a constantspatial light selection device 38. In this case the detection light 24coming from entrance pupil 37 and collimated by objective 34 isspectrally separated by an AOBD (Acousto-Optic Beam Deflector). The AOBDforms a variable spectral grating having a grating distance whichdepends on the applied electrical frequency. By means of the secondobjective 35, the resulting spectral components are focused upon aconstant spatial light selection device 38.

FIG. 12 shows a variable wavelength selection device 30 which isbasically different from the earlier described embodiments inasmuch asit is not based upon the combination of a spectral separation devicewith a spatial selection device. Here, detection light 24 coming fromthe detection light guide 23 is coupled into a light guide 64 withpartially reflecting terminal faces having a refractive index whichdepends on the electrical field intensity. Photoconductor 64 issurrounded by two electrodes 65, 66 to which a variable voltage V can beapplied in order to vary the electrical field strength inside thephotoconductor 64. Based on the Fabry-Perot Effect, the alteration ofthe refractive index in photoconductor 64 caused by the alteration ofthe electrical field strength results in a variation of the optical pathwhich, in turn, causes a light wave selection due to interference.

1. Low-coherence interferometric apparatus for light-optical scanning ofan object (18), by detecting the position of light-remitting sites (20)which are located along a scan path (27) running in a scan direction(28), with a low-coherence interferometer (6) comprising a low-coherentlight source (7), a reference reflector (21) and a detector (25),wherein light emitted from the light source (7) is split by a beamdivider (10) into two optical paths (11, 12), and a first fraction ofthe light is irradiated as measurement light (16) onto the object andreflected at a light-remitting site (20) located at a variable scanposition on the scan path (27), and a second fraction of the light isirradiated as reference light (22) onto the reference reflector (21)where it is reflected, the adjustable scan position is varied along thescan path (27) to perform a scan, and the measurement light (16) and thereference light (22) are combined at a beam junction (10) in such amanner that the resulting detection light (24), upon striking thedetector, generates an interference signal which contains informationabout the reflection intensity of the measurement light relative to therespective scan position, characterized in that a variable wavelengthselection device (30) is positioned in the light path of the detectionlight between the beam junction (10) and the detector (25), by which awavelength-dependent separation of the detection light (24) is performedin such a manner that the detector (25) selectively receives preferablylight with wavelengths which correspond to a predetermined sequence ofwavenumbers k, and different sequences of wavenumbers k can be set forvarying the scan position along the scan path (27).
 2. Apparatusaccording to claim 1, characterized in that, in the spectral range ofthe light source (7), the optical dispersion in the light paths of themeasurement light (16) and the reference light (22) is essentially thesame and the sequence of wavenumbers k is equidistant.
 3. Apparatusaccording to claim 1, characterized in that, in the spectral range ofthe light source (7), the optical dispersion in the light path of themeasurement light (16) differs from the optical dispersion in the lightpath of the reference light (22) and the sequence of wavenumbers kdeviates in such a manner from the equidistant sequence that thedispersion difference is compensated.
 4. Apparatus according to claim 1characterized in that the variable wavelength selection device (30)comprises a spectral separation device (31) by which the detection light(24) is spatially separated, dependent on the wavelength of thedetection light (24), a spatial light selection device (38) having,alternating along a line, light passage areas (39) with lower lightattenuation and light blocking areas (40) with higher light attenuation,the detection light (24) passing with less attenuation through the lightpassage areas (39) than through the blocking areas (40), and an opticalimaging system (34, 35) by which light irradiated from the spectralseparation device (31), is focused upon the spatial light selectiondevice (38), wherein the spreading of the wavelength-dependentseparation of the detection light (24) by the spectral separation device(31) and the distance of the alternating passage and blocking areas (39,40) of the light selection device (38) are variable relative to eachother for setting the sequence of wavenumbers k.
 5. Apparatus accordingto claim 4, characterized in that the angular dispersion of thewavelength-dependent light separation by the spectral separation device(31) is constant and the distance of the alternating light passage andblocking areas (39, 40) of the light selection device (38) is variable.6. Apparatus according to claim 5, characterized in that the spectralseparation device (31) comprises an optical grating (32).
 7. Apparatus,according to claim 4 characterized in that at least on optical element60 of the optical imaging system (36) is simultaneously a component ofthe spectral separation device (31).
 8. Apparatus, according to claim 4characterized in that the spatial light selection device comprises areflective optical element (43), upon which the detection light (24) isirradiated and which selectively provides different reflection in thelight passage areas (39) and in the blocking areas (49).
 9. Apparatusaccording to claim 1 characterized in that the light selection device(38) comprises a rotatable disk (54, 56) with light passage and blockingareas (39, 40) in the form of stripes, running in such a manner that adistance thereof, measured along a line (55) extending over the disksurface, changes during rotation of the disc (54, 55).
 10. Apparatusaccording to claim 1 characterized in that the spatial light selectiondevice (38) comprises an optical element (42, 43, 59) having areflection or transmission which can be selectively adjusted indifferent partial areas thereof by electronic means.
 11. Apparatusaccording to claim 1 characterized in that a light-collecting opticalelement (49) is positioned in the light path of the detection light (24)between the light selection device (38) and the detector (25), in orderto concentrate the detection light (24) on the detector (25).